Semiconductor device

ABSTRACT

A semiconductor device includes a semiconductor region made of a material to which conductive impurities are added, an insulating film formed on a surface of the semiconductor region, and an electroconductive gate electrode formed on the insulating film. The gate electrode is made of a material whose Fermi level is closer to a Fermi level of the semiconductor region than a Fermi level of Si in at least a portion contiguous to the insulating film.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of U.S. patent application Ser. No.15/884,932, filed Jan. 31, 2018, entitled SEMICONDUCTOR DEVICE, whichwas a Continuation of U.S. patent application Ser. No. 15/428,819, filedFeb. 9, 2017, and issued as U.S. Pat. No. 9,911,818 on Mar. 6, 2018,which was a Continuation of U.S. patent application Ser. No. 14/958,867,filed on Dec. 3, 2015, and issued as U.S. Pat. No. 9,601,582 on Mar. 21,2017, which was a Continuation of U.S. patent application Ser. No.14/030,765, filed Sep. 18, 2013, and issued as U.S. Pat. No. 9,224,825on Dec. 29, 2015, which was a Continuation of U.S. patent applicationSer. No. 13/774,549, filed Feb. 22, 2013, and issued as U.S. Pat. No.8,563,981 on Oct. 22, 2013, which was a Continuation of U.S. patentapplication Ser. No. 12/839,983, filed Jul. 20, 2010, and issued as U.S.Pat. No. 8,395,162 on Mar. 12, 2013, the specifications of which areincorporated by reference herein in their entirety. Furthermore, thisapplication claims the benefit of priority of Japanese Application No.2009-170154, filed Jul. 21, 2009, and Japanese Application No.2009-233777, filed Oct. 7, 2009, and Japanese Application No.2010-152085, filed Jul. 2, 2010, the specifications of which areincorporated by reference herein in their entirety.

TECHNICAL FIELD

The present invention relates to a semiconductor device that includes aMISFET (Metal Insulator Semiconductor Field Effect Transistor).

BACKGROUND

For example, a SiC (silicon carbide) semiconductor is excellent indielectric breakdown resistance, thermal conductivity, etc., andtherefore has been receiving attention as a semiconductor suitable to beused in, for example, an inverter of a hybrid vehicle.

FIG. 19 is a schematic sectional view of a conventional SiCsemiconductor device.

A SiC semiconductor device 101 includes an N⁺-type 4H—SiC substrate 102that serves as a foundation of the SiC semiconductor device 101. The SiCsubstrate 102 is made of SiC monocrystal, and is a substrate having anoff-angle at which a surface 121, which is a Si surface defined as aprincipal plane on whose outermost surface Si atoms appear, is inclinedin the direction of a [11-20] axis with respect to a (0001) plane. InFIG. 19, the (0001) plane in the SiC semiconductor device 101 is shownby the broken line.

An N⁻-type epitaxial layer 103 made of SiC doped with a lowerconcentration of N-type impurities than the SiC substrate 102 is stackedon the surface 121 of the SiC substrate 102. The epitaxial layer 103 ismade of SiC that grows from the surface 121 of the SiC substrate 102,and has a principal plane (i.e., surface 117) parallel to the surface121.

A base portion of the epitaxial layer 103 serves as an N⁻-type drainregion 104 being in a constant state without being changed after theepitaxial growth. The epitaxial layer 103 has a P-type body region 105formed on the drain region 104 contiguously with the drain region 104.

Additionally, the epitaxial layer 103 has a gate trench 106 formed bybeing dug downwardly from the surface 117. The gate trench 106penetrates the body region 105 in the layer thickness direction, and itsdeepest portion (i.e., bottom surface 116) reaches the drain region 104.The gate trench 106 is formed in a tapered manner such that the distancebetween a side surface 118A and a side surface 118B that face each otherbecomes narrower in proportion to progress in the depth direction andsuch that the side surfaces 118A and 118B are inclined at taper angle θ₆with respect to a virtual surface S₆ perpendicular to the surface 117 ofthe epitaxial layer 103.

A gate insulating film 107 made of SiO₂ is formed in the gate trench 106so as to cover the whole of the inner surface of the gate trench 106.

The inside of the gate insulating film 107 is filled with polysiliconmaterial (N-type Poly-Si) doped with N-type impurities, and, as aresult, a gate electrode 108 is embedded in the gate trench 106.

An N⁺-type source region 109 is formed at a surface portion of theepitaxial layer 103 on both sides in a direction (rightward-leftwarddirection in FIG. 19) perpendicular to the gate width with respect tothe gate trench 106. The epitaxial layer 103 additionally has a P⁺-typebody contact region 110 that penetrates a center portion of the sourceregion 109 in the direction perpendicular to the gate width from thesurface 117 and that is connected to the body region 105.

An interlayer insulating film 111 made of SiO₂ is stacked on theepitaxial layer 103. Via a contact hole (not shown) formed in theinterlayer insulating film 111, a source wiring 112 is connected to thesource region 109, and a gate wiring 113 is connected to the gateelectrode 108.

A drain wiring 115 is connected to a back surface 118 opposite to thesurface 121 of the SiC substrate 102.

When a voltage exceeding a threshold value is applied to the gateelectrode 108 in a state in which the source wiring 112 is grounded andin which a positive voltage is applied to the drain wiring 115, achannel is formed near an interface with the gate insulating film 107 inthe body region 105, and an electric current flows between the sourcewiring 112 and the drain wiring 115.

SUMMARY

In order to improve the channel mobility of MISFET (i.e., in order toreduce the channel resistance), it is recommended to lower the P-typeimpurity concentration near the surface of a body region in which achannel is formed. However, for example, if the P-type impurityconcentration near the surface of the body region 105 is lowered in theSiC semiconductor device 101, the threshold voltage will fall, andtherefore an off-leakage current flowing between the source wiring 112and the drain wiring 115 is increased in a state in which the SiCsemiconductor device 101 is in an off state (i.e., gate voltage=0 V).

In order to raise the threshold voltage, it is conceivable that thematerial of the gate electrode 108 is changed from N-type Poly-Si toP-type Poly-Si (polysilicon).

FIG. 20A is an energy band view of P-type Poly-Si and P-type SiC. FIG.20B is an energy band view when P-type Poly-Si and P-type SiC are joinedtogether with SiO₂ therebetween.

The work function of N-type Poly-Si is about 4.1 eV, whereas the workfunction qϕ_(m) of P-type Poly-Si is about 5.1 eV as shown in FIG. 20A.The work function q↔ of P-type SiC is about 6.78 eV, and hence thethreshold voltage can be raised by about 1 V in MISFET that uses P-typePoly-Si as the material of the gate electrode 108 in comparison withMISFET 101 that uses N-type Poly-Si as the material of the gateelectrode 108.

However, there is still a difference between the work function qϕ_(m) ofP-type Poly-Si and the work function qχ of P-type SiC, and therefore, ina MIS structure in which P-type Poly-Si (i.e., the gate electrode 108)is joined to the surface of P-type SiC (i.e., the body region 105) withSiO₂ (i.e., the gate insulating film 107) therebetween, the energy bandof P-type SiC bends at its surface as shown in FIG. 20B in a state inwhich a voltage is not applied to P-type Poly-Si (at a time of 0 bias).More specifically, the energy band of P-type SiC bends at the surface ofP-type SiC so that conduction band edge energy Ec_(SiC) approaches aFermi level Ef_(SiC). Therefore, inversion is liable to occur near aninterface with the gate insulating film 107 in the body region 105, andan off-leakage current cannot be satisfactorily reduced.

In FIG. 20A and FIG. 20B, Ec_(Si) designates the conduction band edgeenergy of P-type Poly-Si, and Ef_(Si) designates the Fermi level ofP-type Poly-Si, and Ev_(Si) designates the valence band edge energy ofP-type Poly-Si. Additionally, Ei_(SiC) designates the Fermi level ofintrinsic SiC, and Ev_(SiC) designates the valence band edge energy ofP-type SiC.

FIG. 21 is a view for explaining the off-angle of the SiCmonocrystalline substrate shown in FIG. 19.

The surface 121 of the SiC substrate 102 made of SiC monocrystal is anoff-plane inclined by θ₇ in the direction of the [11-20] axis withrespect to the just (0001) plane. The angle θ₇ is an off-angle of theSiC substrate 102, and, more specifically, is an angle between thenormal [0001] axis of the just (0001) plane and the normal direction n₇of the surface 121 (off-plane).

In the epitaxial growth from the surface on the side of the Si plane ofthe SiC substrate, if the SiC substrate does not have an off-angle ofabout 5°, there is a fear that defects in the crystal of the SiCsubstrate will be easily propagated to the epitaxial layer, and thesedefects in the crystal will lower the withstand voltage of thesemiconductor device. Therefore, conventionally, the withstand voltageof the SiC semiconductor device 101 has been secured by using the SiCsubstrate 102 whose off-angle θ₇ is 5° or more.

On the other hand, from the viewpoint of securing high channel mobilityin the SiC semiconductor device 101, it is preferable to allow each ofthe side surfaces 118A and 118B of the gate trench 106 in which achannel is formed to be a (11-20) plane perpendicular to the [11-20]axis.

However, the side surfaces 118A and 118B of the gate trench 106 formedin the epitaxial layer 103 having an off-angle are inclined by off-angleθ₇ with respect to the (11-20) plane, and therefore it is difficult tomaintain positional relationships therebetween so as to be parallelthereto. Moreover, the gate trench 106 has taper angle θ₆, and thereforethe inclination angle with respect to the (11-20) plane of the sidesurface 118A that is one of the two side surfaces becomes greater bytaper angle θ₆ than off-angle θ₇. As a result, disadvantageously, thechannel mobility in the side surface 118A falls. Additionally,disadvantageously, when the off-angle is close to 0°, the impurityconcentration (carrier concentration) of the epitaxial layer 103 isexcessive, and the surface 117 of the epitaxial layer 103 becomes rough.

It is an object of the present invention to provide a semiconductordevice capable of improving channel mobility and capable of furtherlowering an off-leakage current.

It is another object of the present invention to provide a semiconductordevice capable of improving channel mobility while securing withstandvoltage and capable of correcting the imbalance (ununiformity) inchannel characteristics by a plane orientation.

The foregoing or other objects, features, and effects of the presentinvention will be apparent from a description of embodiments hereinaftergiven with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding, reference is now made to thefollowing description taken in conjunction with the accompanyingDrawings in which:

FIG. 1 is a schematic plan view of a semiconductor device according to afirst embodiment of the present invention;

FIG. 2 is a schematic sectional view of the semiconductor device alongline II-II of FIG. 1;

FIG. 3A is an energy band view of P-type Poly-SiC and P-type SiC;

FIG. 3B is an energy band view when P-type Poly-SiC and P-type SiC arejoined together with SiO2 placed therebetween;

FIG. 4A to FIG. 4E are schematic sectional views for explaining amanufacturing method of the semiconductor device in order of steps;

FIG. 5 is a schematic sectional view of a semiconductor device accordingto a second embodiment of the present invention;

FIG. 6 is a schematic sectional view of a semiconductor device accordingto a third embodiment of the present invention;

FIG. 7 is a schematic sectional view of a semiconductor device accordingto a fourth embodiment of the present invention;

FIG. 8 is a schematic view showing a unit cell having a crystallinestructure of 4H—SiC;

FIG. 9 is a view for explaining an off-angle of a SiC monocrystallinesubstrate shown in FIG. 7;

FIG. 10A is an enlarged view of a main part of one side surface of agate trench of a conventional SiC semiconductor device;

FIG. 10B is an enlarged view of a main part of the other side surface ofthe gate trench of the conventional SiC semiconductor device;

FIG. 11A is an enlarged view of a main part of one side surface of agate trench of the semiconductor device shown in FIG. 7;

FIG. 11B is an enlarged view of a main part of the other side surface ofthe gate trench of the semiconductor device shown in FIG. 7;

FIG. 12 is a schematic sectional view of a semiconductor deviceaccording to a fifth embodiment of the present invention;

FIG. 13 is a schematic sectional view of MOSFET in an example and in acomparative example;

FIG. 14 is a graph showing a relationship between carrier concentrationand an off-angle;

FIG. 15 is a graph showing a relationship between surface roughness ofan epitaxial layer and an off-angle;

FIG. 16 is a schematic plan view of test wafers of Example 7 andComparative Example 2;

FIG. 17A is a view showing the magnitude of drain current Ids of Example7;

FIG. 17B is a view showing the magnitude of drain current Ids ofComparative Example 2;

FIG. 18A is a view showing the magnitude of gate threshold voltage Vthof Example 7;

FIG. 18B is a view showing the magnitude of gate threshold voltage Vthof Comparative Example 2;

FIG. 19 is a schematic sectional view of a conventional SiCsemiconductor device;

FIG. 20A is an energy band view of P-type Poly-Si and P-type SiC;

FIG. 20B is an energy band view when P-type Poly-Si and P-type SiC arejoined together with SiO2 placed therebetween; and

FIG. 21 is a view for explaining an off-angle of a SiC monocrystallinesubstrate shown in FIG. 19.

DETAILED DESCRIPTION

Referring now to the drawings, wherein like reference numbers are usedherein to designate like elements throughout, the various views andembodiments of a semiconductor device are illustrated and described, andother possible embodiments are described. The figures are notnecessarily drawn to scale, and in some instances the drawings have beenexaggerated and/or simplified in places for illustrative purposes only.One of ordinary skill in the art will appreciate the many possibleapplications and variations based on the following examples of possibleembodiments.

A semiconductor device according to an aspect of the present inventionincludes a semiconductor region made of materials to which conductiveimpurities are added, an insulating film formed on the surface of thesemiconductor region, and a gate electrode formed on the insulatingfilm. The gate electrode is made of a material in which a Fermi level ofat least a portion contiguous to the insulating film is closer to aFermi level of the semiconductor region than a Fermi level of Si, andthe gate electrode is electrically conductive.

In a structure in which a trench is formed by being dug downwardly fromthe upper surface of the semiconductor region and in which a gateelectrode is embedded in the trench, the inner surface (side surface andbottom surface) of the trench is included in the concept of the surfaceof the semiconductor region.

In MISFET in which a surface portion facing the insulating film in thesemiconductor region serves as a channel region, channel mobility can beimproved by lowering the concentration of conductive impurities of thechannel region. However, if the concentration of impurities of thechannel region is lowered, the threshold voltage of MISFET will fall.

Therefore, a material whose Fermi level is closer to the Fermi level ofthe semiconductor region than the Fermi level of Si is employed as thematerial of at least a portion contiguous to the insulating film in thegate electrode. Accordingly, in a MIS structure formed of thesemiconductor region, the insulating film, and the gate electrode, thebend of an energy band of the semiconductor region in a state in which avoltage is not applied to the gate electrode can be reduced incomparison with a case in which Si is employed as the material of thegate electrode. In other words, it can be brought close to an ideal MISstructure. As a result, the threshold voltage of MISFET can be raised,and the off-leakage current can be reduced in comparison with a case inwhich Si is employed as the material of the gate electrode.

Therefore, an improvement in channel mobility and a further decrease inoff-leakage current can be achieved.

Preferably, a material having a wider band gap than the band gap of Siis employed as the material of the semiconductor region. Accordingly,the dielectric breakdown strength of the semiconductor region can beraised.

Additionally, preferably, in a case in which a material having a widerband gap than the band gap of Si is employed as the material of thesemiconductor region, the material of the gate electrode has a widerband gap than the band gap of Si. As a result, the bend of the energyband of the semiconductor region in the MIS structure can be madesmaller.

Additionally, the material of the semiconductor region may be the sameas the material of the gate electrode. For example, the material of thesemiconductor region may be P-type SiC, and the material of the gateelectrode may be P-type Poly-SiC. Accordingly, the energy band structureof the semiconductor region and the energy band structure of the gateelectrode can be approximated to each other. As a result, the bend ofthe energy band of the semiconductor region in the MIS structure can bemade even smaller.

In the present invention, the identity of materials is not hindered bywhether impurities have been added to the materials and whether thematerials are crystalline (monocrystalline or polycrystalline) ornoncrystalline. For example, P-type SiC and N-type Poly-SiC(polycrystalline silicon carbide) are the same in material as eachother.

The gate electrode may be made of a material whose Fermi level is closerto the Fermi level of the semiconductor region than the Fermi level ofSi in its whole. Preferably, in this case, the material of the gateelectrode is the same as the material of the semiconductor region.Preferably, the material of the gate electrode is P-type Poly-SiC, forexample, if the material of the semiconductor region is P-type SiC.

Additionally, the gate electrode may be made of a material whose Fermilevel is closer to the Fermi level of the semiconductor region than theFermi level of Si in only a portion contiguous to the insulating film.For example, the gate electrode may include an electroconductive firstlayer that is contiguous to the insulating film and that is made of amaterial whose Fermi level is closer to the Fermi level of thesemiconductor region than the Fermi level of Si, and anelectroconductive second layer that is stacked on the first layer andthat is made of a material different from the material of the firstlayer. Preferably, in this case, the first layer is P-type Poly-SiC, andthe second layer is P-type Poly-SiC.

Additionally, preferably, if the semiconductor region and/or the gateelectrode contain a P-type impurity, the impurity is B (boron).

Additionally, preferably, the semiconductor region has an impurityconcentration of 1×10¹⁸ cm⁻³ or less in its portion located at a depthof 1000 Å or less from the insulating film. Accordingly, channelmobility can be improved in MISFET in which a surface portion facing theinsulating film in the semiconductor region serves as a channel region.

Additionally, preferably, if the semiconductor region is formed on asemiconductor substrate, the off-angle of the semiconductor substrate isless than 4° exceeding 0°.

A semiconductor device according to another aspect of the presentinvention includes a SiC substrate, an epitaxial layer formed on onesurface side of the SiC substrate, and a gate trench formed to be dugdownwardly from the principal plane of the epitaxial layer, and theoff-angle of the SiC substrate is less than 4° exceeding 0°.

The SiC substrate has one surface serving as an off-plane inclined inthe direction of the [11-20] axis at less than 4° exceeding 0° withrespect to a just (000-1) plane. The angle less than 4° exceeding 0° isthe off-angle of the SiC substrate, and is an angle between, forexample, the normal [0001] axis of the just (0001) plane and the normaldirection of the one surface (off-plane). An epitaxial layer is formedby SiC that grows from the one surface of the SiC substrate, andtherefore the epitaxial layer has a principal plane parallel to the onesurface of the SiC substrate.

Therefore, an inclination angle of the side surface of the gate trenchwith respect to the (11-20) plane perpendicular to the [11-20] axisbecomes smaller than the angle of the side surface of the gate trenchformed in the epitaxial layer that has an off-angle of 4° or more.Accordingly, the positional relationship of the side surface of the gatetrench with respect to the (11-20) plane can be brought close to aparallel relationship, and therefore channel mobility can be improved.Additionally, the imbalance (ununiformity) of channel characteristics bya plane orientation can be controlled. Additionally, an appropriateimpurity concentration and flatness of the epitaxial layer can bemaintained.

Preferably, one surface of the SiC substrate is a C plane. In this case,the bottom surface of the gate trench is a surface inclined in thedirection of the [11-20] axis by an off-angle with respect to the just(000-1) plane. Therefore, when a semiconductor device is produced, theoxidation of the bottom surface and the side surface of the gate trenchproceeds under the condition that the oxidation rate of the bottomsurface of the gate trench and the oxidation rate of the side surfacesatisfy the relational expression: oxidation rate of the bottomsurface/oxidation rate of the side surface >1. As a result, a gateinsulating film in which, for example, a portion on the bottom surfaceis thicker than a portion on the side surface can be formed. Therefore,the dielectric breakdown of the portion on the bottom surface can beprevented, and withstand voltage can be improved by appropriatelydesigning the thickness of the portion on the bottom surface in the gateinsulating film.

Preferably, the off-angle of the SiC substrate is 0.3° or more and isless than 4°, and, more preferably, is 1°. Accordingly, an electriccurrent (drain current) flowing through the channel and a voltage (gatethreshold voltage) produced when the electric current begins to flow canbe fixed at substantially constant magnitudes, respectively, in allplane orientations without causing variations by the plane orientationof the sidewall of the gate trench. The gate trench may be formed in atapered manner.

Preferably, if a body region is formed beside the gate trench in theepitaxial layer and if a gate electrode facing the body region with agate insulating film therebetween is formed on the side surface of thegate trench, the gate electrode is formed by using the same material asthe body region.

In this case, the whole of the gate electrode may be made of the samematerial as the body region. For example, if the material of the bodyregion is P-type SiC, the material of the gate electrode may be P-typePoly-SiC.

Additionally, only a portion of the gate electrode contiguous to thegate insulating film may be made of the same material as the bodyregion. For example, if the material of the body region is P-type SiC,the gate electrode may have a layered structure consisting of a firstlayer that is contiguous to the gate insulating film and made of P-typePoly-SiC and a second layer that is stacked on the first layer and madeof P-type Poly-Si.

Preferably, the body region has an impurity concentration of 1×10¹⁸ cm⁻³or less in its portion located at a depth of 1000 Å or less from thegate insulating film. Accordingly, the mobility of the channel formed inthe body region can be further improved.

A semiconductor device according to still another aspect of the presentinvention includes a first conductivity type SiC substrate, a firstconductivity type epitaxial layer formed on a side of one surface of theSiC substrate, a second conductivity type body region formed on asurface portion of the epitaxial layer, a gate trench formed in theepitaxial layer so as to penetrate the body region from the surface ofthe epitaxial layer, a gate insulating film formed on an inner surfaceof the gate trench, and a gate electrode formed on the gate insulatingfilm. The off-angle of the SiC substrate is less than 4° exceeding 0°,and the gate electrode is made of the same material as the body region.

The SiC substrate has one surface serving as an off-plane inclined inthe direction of the [11-20] axis at less than 4° exceeding 0° withrespect to a just (000-1) plane. The angle less than 4° exceeding 0° isthe off-angle of the SiC substrate, and is an angle between, forexample, the normal [0001] axis of the just (0001) plane and the normaldirection of the one surface (off-plane). An epitaxial layer is formedby SiC that grows from the one surface of the SiC substrate, andtherefore the epitaxial layer has a principal plane parallel to the onesurface of the SiC substrate.

Therefore, an inclination angle of the side surface of the gate trenchwith respect to the (11-20) plane perpendicular to the [11-20] axisbecomes smaller than the angle of the side surface of the gate trenchformed in the epitaxial layer that has an off-angle of 4° or more.Accordingly, the positional relationship of the side surface of the gatetrench with respect to the (11-20) plane can be brought close to aparallel relationship, and therefore channel mobility can be improved.Additionally, the imbalance (ununiformity) of channel characteristics bya plane orientation can be controlled. Additionally, an appropriateimpurity concentration and flatness of the epitaxial layer can bemaintained.

The material of the body region is the same as the material of the gateelectrode. For example, the material of the body region is P-type SiC,and the material of the gate electrode is P-type Poly-SiC. Accordingly,the energy band structure of the body region and the energy bandstructure of the gate electrode can be approximated to each other. As aresult, the bend of the energy band of the body region in the MISstructure can be made small. In other words, it can be brought close toan ideal MIS structure. As a result, the threshold voltage of MISFET canbe raised, and the off-leakage current can be reduced in comparison witha case in which Si is employed as the material of the gate electrode.Therefore, an improvement in channel mobility and a further decrease inoff-leakage current can be achieved.

Preferably, the body region has an impurity concentration of 1×10¹⁸ cm⁻³or less in its portion located at a depth of 1000 Å or less from thegate insulating film. Accordingly, the mobility of the channel formed inthe body region can be further improved.

Preferably, if it is supposed that the gate insulating film is dividedinto a first portion on the bottom surface of the gate trench and asecond portion on the side surface of the gate trench, the first portionis thicker than the second portion. Accordingly, the dielectricbreakdown of the portion on the bottom surface can be prevented, andwithstand voltage can be improved.

Hereinafter, embodiments of the present invention will be concretelydescribed with reference to the accompanying drawings.

First to Third Embodiments

FIG. 1 is a schematic plan view of a semiconductor device according to afirst embodiment of the present invention. FIG. 2 is a schematicsectional view of the semiconductor device along line II-II of FIG. 1.

The semiconductor device 1 has a quadrangular external shape(substantially square shape) when viewed planarly as shown in FIG. 1.

The semiconductor device 1 includes a semiconductor substrate 2 as shownin FIG. 2. The semiconductor substrate 2 is made of, for example, N-typeSiC doped with N-type impurities. The semiconductor substrate 2 has anoff-angle of, for example, less than 4° exceeding 0°. The semiconductorsubstrate 2 may consist of a monolayer of N-type SiC, or may be formedby stacking an N-type SiC layer on the N-type SiC substrate (forexample, by epitaxial growth).

A plurality of P-type body regions (well regions) 3 are formed at asurface portion of the semiconductor substrate 2. The body regions 3each have a quadrangular shape (substantially square shape) when viewedplanarly, and are arranged in a matrix manner. The depth of each bodyregion 3 is, for example, 5000 Å to 6500 Å (500 nm to 650 nm). Each bodyregion 3 has an impurity concentration profile in which the P-typeimpurity concentration of a portion whose depth is 1000 Å (100 nm) orless based on the center in the thickness direction of a gate insulatingfilm 6 described later is 1×10¹⁸ cm⁻³ or less.

An N-type source region 4 is formed at a surface portion of each bodyregion 3 with a gap with respect to the peripheral edge of the bodyregion 3.

A P⁺-type body contact region 5 doped with a higher concentration ofP-type impurities than the body region 3 is formed inside each sourceregion 4. Each body contact region 5 is formed to penetrate the sourceregion 4 in the depth direction.

The gate insulating film 6 is formed on the surface of the semiconductorsubstrate 2. The gate insulating film 6 is made of, for example, SiO₂.The thickness of the gate insulating film 6 is, for example, about 400 Å(40 nm).

A gate electrode 7 is formed on the gate insulating film 6. The gateelectrode 7 is formed like a grid when viewed planarly as shown inFIG. 1. In FIG. 1, the gate electrode 7 is shown in such a way as to seethrough an interlayer insulating film 8 and a source metal 10 describedlater. The gate electrode 7 is made of P-type Poly-SiC doped with B(i.e., boron) that is a P-type impurity.

The interlayer insulating film 8 is formed on the semiconductorsubstrate 2 as shown in FIG. 2. The surface of the semiconductorsubstrate 2, as well as the gate electrode 7, is covered with theinterlayer insulating film 8. The interlayer insulating film 8 is madeof, for example, SiO₂.

The interlayer insulating film 8 has a contact hole 9 formed at aposition facing each body contact region 5. Each contact hole 9penetrates the gate insulating film 6, and the whole area of the bodycontact region 5 and a portion around the body contact region 5 in thesource region 4 face the inside of each contact hole 9.

A source metal 10 is formed on the interlayer insulating film 8. Thesource metal 10 enters each contact hole 9 formed in the interlayerinsulating film 8, and is connected to the source region 4 and the bodycontact region 5. The source metal 10 is made of, for example, ametallic material whose principal component is Al. To achieve the ohmiccontact of the source metal 10 with respect to the source region 4 andthe body contact region 5, an ohmic metal made of Ni may be formed onthe lower layer of the source metal 10.

On the center of a portion along one sideline of the semiconductorsubstrate 2, the interlayer insulating film 8 and the source metal 10are selectively removed, and, as a result, an opening that exposes aportion of the gate electrode 7 as a gate pad 11 used to be connected tothe outside is formed as shown in FIG. 1.

A drain metal 12 is formed on the entire surface of the back side of thesemiconductor substrate 2.

In a state in which the source metal 10 is grounded and in which anappropriate positive voltage is applied to the drain metal 12, theelectric potential (gate voltage) of the gate electrode 7 is controlled,and, as a result, a channel is formed near an interface with the gateinsulating film 6 in the body region 3, and an electric current flowsbetween the source metal 10 and the drain metal 12.

FIG. 3A is an energy band view of P-type Poly-SiC and P-type SiC. FIG.3B is an energy band view when P-type Poly-SiC and P-type SiC are joinedtogether with SiO₂ placed therebetween.

As shown in FIG. 3A, the work function qχ of P-type SiC that is thematerial of the body region 3 is about 6.78 eV. On the other hand, thework function qϕ_(m) of P-type Poly-SiC that is the material of the gateelectrode 7 is 5.5 eV to 7.0 eV. The work function of P-type Poly-Si isabout 5.1 eV. Therefore, the Fermi level Ef_(poly) of P-type Poly-SiC iscloser to the Fermi level Ef_(SiC) of P-type SiC than the Fermi level ofSi.

Therefore, in a MOS structure in which the gate electrode 7 is joined tothe surface of the body region 3 with SiO₂ that is the material of thegate insulating film 6 and that is placed therebetween, the bend of theenergy band of the body region 3 is hardly made in a state (at a time of0 bias) in which a voltage is not applied to the gate electrode 7 asshown in FIG. 3B. In other words, the semiconductor device 1 has a MOSstructure close to an ideal MOS structure. Therefore, in thesemiconductor device 1, an inversion does not easily occur near aninterface with the gate insulating film 6 in the body region 3 in astate in which a voltage is not applied to the gate electrode 7.

In FIG. 3A and FIG. 3B, Ec_(poly) designates the conduction band edgeenergy of P-type Poly-SiC, and Ev_(poly) designates the valence bandedge energy of P-type Poly-SiC. Ec_(SiC) designates the conduction bandedge energy of P-type SiC, and Ev_(sic) designates the valence band edgeenergy of P-type SiC. Ei_(sic) designates the Fermi level of intrinsicSiC.

As mentioned above, P-type Poly-SiC whose Fermi level Ef_(Poly) iscloser to the Fermi level of the body region 3 than the Fermi level ofSi is employed as the material of the gate electrode 7. Accordingly, inthe MOS (MIS) structure formed of the body region 3, the gate insulatingfilm 6, and the gate electrode 7, the bend of the energy band of thebody region 3 in a state in which a voltage is not applied to the gateelectrode 7 can be reduced in comparison with a case in which Si isemployed as the material of the gate electrode 7. As a result, thethreshold voltage of MOSFET of the semiconductor device 1 can be raised,and an off-leakage current can be reduced in comparison with a case inwhich Si is employed as the material of the gate electrode 7.

The P-type impurity concentration of a surface portion facing the gateinsulating film 6 in the body region 3, i.e., the P-type impurityconcentration of the channel region is set at a low concentration of1×10¹⁸ cm⁻³ or less, and therefore channel mobility can be improved.

Therefore, in the semiconductor device 1, an improvement in channelmobility and a decrease in off-leakage current can be achieved.

SiC has a wider band gap than the band gap of Si. Therefore, dielectricbreakdown strength can be raised by employing SiC as the material of thesemiconductor substrate 2 (i.e., body region 3) in comparison with acase in which Si is employed as the material of the semiconductorsubstrate 2.

FIG. 4A to FIG. 4E are schematic sectional views for sequentiallyexplaining the steps of a manufacturing process of the semiconductordevice.

In the manufacturing process of the semiconductor device 1, first, Althat is a P-type impurity used to form the body region 3 is selectivelyinjected (implanted) into the surface portion of the semiconductorsubstrate 2 as shown in FIG. 4A. Furthermore, Al that is a P-typeimpurity used to form the body contact region 5 is selectively injectedinto the surface portion of the body region 3. Furthermore, P(phosphorus) that is an N-type impurity used to form the source region 4is selectively injected into the surface portion of the body region 3.Thereafter, annealing is performed, and the body region 3, the sourceregion 4, and the body contact region 5 are formed at the surfaceportion of the semiconductor substrate 2.

Next, as shown in FIG. 4B, the gate insulating film 6 is formed on thesurface of the semiconductor substrate 2 according to a thermaloxidation method.

Thereafter, as shown in FIG. 4C, Poly-SiC is deposited with a thicknessof about 5000 Å (500 nm) on the gate insulating film 6 according to aCVD (Chemical Vapor Deposition) method. Next, a deposition layer 13 ofPoly-SiC is doped with B in order to change the deposition layer 13 ofPoly-SiC to the deposition layer of P-type Poly-SiC. The doping by useof B is achieved by an ion implantation method in which, for example,the injection energy is 100 keV and the dose amount is 2×10¹⁵ cm⁻².After the doping by use of B, annealing is performed to activate this B.The annealing temperature is, for example, 1600° C.

Next, as shown in FIG. 4D, the deposition layer of P-type Poly-SiC isselectively removed, and the gate electrode 7 made of P-type Poly-SiC isformed on the gate insulating film 6 according to photolithography andetching.

Next, as shown in FIG. 4E, the interlayer insulating film 8 is formed onthe semiconductor substrate 2 according to the CVD method. Furthermore,the contact hole 9, which penetrates the interlayer insulating film 8and the gate insulating film 6, is formed according to photolithographyand etching.

Thereafter, the source metal 10 is formed on the interlayer insulatingfilm 8 according to a sputtering method. Furthermore, the gate pad 11 isformed according to photolithography and etching. Furthermore, the drainmetal 12 is formed on the back side of the semiconductor substrate 2according to the sputtering method. The semiconductor device 1 of FIG. 1can be obtained in this way.

FIG. 5 is a schematic sectional view of a semiconductor device accordingto a second embodiment of the present invention. In FIG. 5, the samereference numeral as in FIG. 2 is given to an element equivalent to eachelement of FIG. 2. When a structure shown in FIG. 5 is described, onlydifferences between the structure of FIG. 2 and that of FIG. 5 aredescribed, and the element to which the same reference numeral as inFIG. 2 is given is not described hereinafter.

In the semiconductor device 14 shown in FIG. 5, the gate electrode 15formed on the gate insulating film 6 has a two-layer structureconsisting of a first layer 16 contiguous to the gate insulating film 6and a second layer 17 stacked on the first layer 16.

The first layer 16 is made of P-type Poly-SiC whose Fermi level iscloser to the Fermi level of the body region 3 than the Fermi level ofSi.

The second layer 17 is made of a material different from that of thefirst layer 16. More specifically, the second layer 17 is made of P-typePoly-Si, or N-type Poly-Si, or a metal.

The same operation and effect as the semiconductor device 1 of FIG. 2can be fulfilled by the structure of the semiconductor device 14.

FIG. 6 is a schematic sectional view of a semiconductor device accordingto a third embodiment of the present invention.

Although the semiconductor device 1 of FIG. 2 and the semiconductordevice 14 of FIG. 5 have a vertical MOSFET (Metal Oxide SemiconductorField Effect Transistor) that employs a planar gate structure, thesemiconductor device 18 of FIG. 6 has a MOSFET that employs a trenchgate structure.

The semiconductor device 18 includes an N-type SiC substrate 19 made ofN⁺-type SiC.

An N-type SiC layer 20 made of N⁻-type SiC is formed on the N-type SiCsubstrate 19 by epitaxial growth. A base portion of the N-type SiC layer20 remains without changes after epitaxial growth, and serves as anN⁻-type drain region 21. A surface portion of the N-type SiC layer 20 isallowed to serve as a P-type body region (well region) 22 by being dopedwith P-type impurities.

A gate trench 23 is dug into the N-type SiC layer 20 downwardly from itssurface. In the same way as, for example, the gate electrode 7 shown inFIG. 1, the gate trench 23 is formed like a grid when viewed planarly.The gate trench 23 penetrates the body region 22, and its deepestportion reaches the drain region 21.

A gate insulating film 24 is formed on the inner surface of the gatetrench 23. The gate insulating film 24 is made of, for example, SiO₂.The thickness of the gate insulating film 24 is, for example, about 400Å (40 nm).

The inside of the gate insulating film 24 is filled with P-type Poly-SiCdoped with B (boron) that is a P-type impurity, and, as a result, a gateelectrode 25 made of this P-type Poly-SiC is embedded in the gate trench23.

An N⁺-type source region 26 is formed at a surface portion of the bodyregion 22.

Additionally, a P⁺-type body contact region 27 penetrating the sourceregion 26 in the thickness direction is formed at the surface portion ofthe body region 22 at a position with a space with respect to the gatetrench 23 in each region surrounded by the gate trench 23.

An interlayer insulating film 28 is stacked on the N-type SiC layer 20.The interlayer insulating film 28 is made of, for example, SiO₂.

The interlayer insulating film 28 has a contact hole 29 penetratingtherethrough at a position facing each body contact region 27. The wholearea of the body contact region 27 and a portion around the body contactregion 27 in the source region 26 face the inside of each contact hole29.

A source metal 30 is formed on the interlayer insulating film 28. Thesource metal 30 enters each contact hole 29, and is connected to thesource region 26 and the body contact region 27. The source metal 30 ismade of, for example, a metallic material whose principal component isAl. To achieve the ohmic contact of the source metal 30 with respect tothe source region 26 and the body contact region 27, an ohmic metal madeof Ni may be formed on the lower layer of the source metal 30.

A drain metal 31 is formed on the entire surface of the back side of theN-type SiC substrate 19. The drain metal 31 is made of, for example, ametallic material whose principal component is Al. To achieve the ohmiccontact of the drain metal 31 with respect to the N-type SiC substrate19, an ohmic metal 32 made of Ni may be formed between the N-type SiCsubstrate 19 and the drain metal 31 as shown in FIG. 6.

In a state in which the source metal 30 is grounded and in which anappropriate positive voltage is applied to the drain metal 31, theelectric potential (gate voltage) of the gate electrode 25 iscontrolled, and, as a result, a channel is formed near an interface withthe gate insulating film 24 in the body region 22, and an electriccurrent flows between the source metal 30 and the drain metal 31.

The same operation and effect as the semiconductor devices 1 and 14 canbe fulfilled by the structure of the semiconductor device 18.

Although the first to third embodiments of the present invention havebeen described as above, the present invention can be embodied in otherforms.

Without being limited to P-type Poly-SiC, materials whose Fermi level iscloser to the Fermi level of the body regions 3 and 22 than the Fermilevel of Si can be widely employed, for example, as the material of thegate electrode 7, as the material of the first layer 16, and as thematerial of the gate electrode 25.

Preferably, materials whose band gap is wider than the band gap of Siare employed as the material of the gate electrode 7, as the material ofthe first layer 16, and as the material of the gate electrode 25.Examples of materials suitable as the material of the gate electrode 7,as the material of the first layer 16, and as the material of the gateelectrode 25 include SiC doped with N-type impurities or P-typeimpurities, diamond doped with N-type impurities or P-type impurities,BN (boron nitride) doped with N-type impurities or P-type impurities,AlN (aluminum nitride) doped with N-type impurities or P-typeimpurities, GaN (gallium nitride) doped with N-type impurities or P-typeimpurities, GaAs (gallium arsenide) doped with N-type impurities orP-type impurities, GaP (gallium phosphide) doped with N-type impuritiesor P-type impurities, and ZnO (zinc oxide) doped with N-type impuritiesor P-type impurities.

SiC, diamond, BN, AlN, GaN, GaAs, GaP, and ZnO may be monocrystalline,or polycrystalline, or amorphous. If these materials are polycrystallineor amorphous, there is an advantage in the fact that their films can bemore easily formed than monocrystalline materials.

B, Al, Ga (gallium), and In (indium) can be mentioned as P-typeimpurities with which SiC and diamond are doped.

N (nitrogen), P (phosphorus), As (arsenic), and Sb (antimony) can bementioned as N-type impurities with which SiC and diamond are doped.

Be (beryllium), Mg (magnesium), Ca (calcium), and Sr (strontium) can bementioned as P-type impurities with which BN, AlN, GaN, GaAs, GaP, andZnO are doped.

C (carbon), Si, Ge (germanium), and Sn (tin) can be mentioned as N-typeimpurities with which BN, AlN, GaN, GaAs, GaP, and ZnO are doped.

Although a structure having a vertical MOSFET has been provided as anexample in the above embodiments, the present invention can be appliedto a structure having a lateral MOSFET.

Fourth and Fifth Embodiments

FIG. 7 is a schematic sectional view of a semiconductor device accordingto a fourth embodiment of the present invention.

The semiconductor device 41 has a structure in which a plurality of unitcells of a trench-gate type VDMOSFET are arranged. In FIG. 7, a part ofthe unit cells is shown.

The semiconductor device 41 includes a 4H—SiC substrate 42 that servesas a foundation of the semiconductor device 41. The SiC substrate 42 ismade of SiC monocrystal, and is a substrate having an off-angle at whicha surface 421, which is a C plane defined as a principal plane (surface421) on whose outermost surface C-atoms appear, is inclined in thedirection of a [11-20] axis with respect to a (0001) plane. The SiCsubstrate 42 is doped with a high concentration of N-type impurities.The N-type impurity concentration is, for example, 1×10¹⁷ cm⁻³ or more,and, preferably, 1×10¹⁸ cm⁻³ to 1×10²¹ cm⁻³. In FIG. 7, the (000-1)plane in the semiconductor device 41 is shown by the broken line.

An N⁻-type epitaxial layer 43 made of SiC doped with a lowerconcentration of N-type impurities than the SiC substrate 42 is stackedon the surface 421 of the SiC substrate 42. The epitaxial layer 43 ismade of SiC that grows from the surface 421 of the SiC substrate 42, andhas a principal plane (i.e., surface 431) parallel to the surface 421.

A base portion of the epitaxial layer 43 serves as an N⁻-type drainregion (drift region) 44 whose whole remains without changes afterepitaxial growth. The N-type impurity concentration of the drain region44 is, for example, 1×10¹⁵ cm⁻³ to 1×10¹⁷ cm⁻³.

On the other hand, a P-type body region 45 is formed at a surfaceportion of the epitaxial layer 43. The body region 45 is contiguous tothe drain region 44. The P-type impurity concentration of the bodyregion 45 is, for example, 1×10¹⁶ cm⁻³ to 1×10¹⁹ cm⁻³.

A gate trench 46 is dug into the epitaxial layer 43 downwardly from thesurface 431. The gate trenches 46 (not shown in FIG. 7) are formed atpredetermined intervals therebetween, and are extended in parallel witheach other in the same direction (which is a direction perpendicular tothe sheet surface of FIG. 7 and which can be hereinafter referred to asa “direction along the gate width”), and, for example, have a stripestructure.

The gate trench 46 penetrates the body region 45 in the layer thicknessdirection, and its deepest portion (bottom surface 461) reaches thedrain region 44. The gate trench 46 is formed in a tapered manner suchthat the distance between a side surface 462A and a side surface 462Bthat face each other becomes narrower in proportion to progress in thedepth direction. For example, an inclination angle θ₁ (taper angle θ₁)of the side surfaces 462A and 462B with respect to a virtual surface S₁perpendicular to the surface 431 of the epitaxial layer 43 is 0° to 50°,preferably, 0° to 1°.

A gate insulating film 47 made of SiO₂ is formed on the inner surface(bottom surface 461, side surfaces 462A and 462B) of the gate trench 46and on the surface 431 of the epitaxial layer 43 so as to cover thewhole of the inner surface of the gate trench 46.

The gate insulating film 47 integrally has a bottom portion 471 placedon the bottom surface 461 of the gate trench 46 and a side portion 472placed on the side surfaces 462A and 462B of the gate trench 46.Thickness t₁ of the bottom portion 471 is greater than thickness t₂ ofthe side portion 472. For example, t₁ is 0.1 μm to 2 μm, and t₂ is 0.05μm to 0.2 μm.

The body region 45 has a P-type impurity concentration of 1×10¹⁸ cm⁻³ orless (preferably, 7×10¹⁷ cm⁻³ to 9×10¹⁷ cm⁻³) in a region 451 whosedepth d₁ from the center in the thickness direction of the side portion472 of the gate insulating film 47 is 1000 Å or less.

The inside of the gate insulating film 47 is filled with a polysiliconmaterial (P-type Poly-SiC) doped with P-type impurities, and, as aresult, a gate electrode 48 is embedded in the gate trench 46.

An N⁺-type source region 49 is formed at a surface portion of the bodyregion 45 on both sides in a direction (rightward-leftward direction inFIG. 7) perpendicular to the gate width with respect to the gate trench46. The source region 49 is a region doped with a high concentration ofN-type impurities, higher than the N-type impurity concentration of thedrain region 44. The N-type impurity concentration of the source region49 is, for example, 1×10¹⁸ cm⁻³ to 1×10²¹ cm⁻³. The source region 49extends in the direction along the gate width at a position contiguousto the gate trench 46, and its bottom portion is contiguous to the bodyregion 45.

The epitaxial layer 43 additionally has a P⁺-type body contact region 50that penetrates a center portion of the source region 49 in thedirection perpendicular to the gate width from the surface 431 and thatis connected to the body region 45. The body contact region 50 is aregion doped with a high concentration of P-type impurities, higher thanthe P-type impurity concentration of the body region 45. The P-typeimpurity concentration of the body contact region 50 is, for example,1×10¹⁸ cm⁻³ to 1×10²¹ cm⁻³.

In other words, the gate trench 46 and the source region 49 arealternately disposed in the direction perpendicular to the gate width,and extend in the direction along the gate width. The boundary betweenunit cells contiguous in the direction perpendicular to the gate widthis set along the source region 49 on the source region 49. At least onebody contact region 50 is provided in such a way as to stretch over bothunit cells contiguous in the direction perpendicular to the gate width.The boundary between unit cells contiguous in the direction along thegate width is set so that the gate electrode 48 contained in each unitcell has a predetermined gate width.

An interlayer insulating film 51 made of SiO₂ is stacked on theepitaxial layer 43. A source wiring 52 is connected to the source region49 via a contact hole (not shown) formed in the interlayer insulatingfilm 51. The source wiring 52 is grounded.

A gate wiring 54 is connected to the gate electrode 48 via anothercontact hole (not shown) formed in the interlayer insulating film 51.

A drain wiring 56 is connected to a back surface 422 opposite to thesurface 421 of the SiC substrate 42.

A predetermined voltage (voltage greater than the gate thresholdvoltage) is applied to the gate wiring 54 in a state in which apredetermined potential difference is generated between the sourcewiring 52 and the drain wiring 56 (in the source-drain), and, as aresult, a channel is formed near an interface with the gate insulatingfilm 47 in the body region 45 by means of an electric field from thegate electrode 48. As a result, an electric current flows between thesource wiring 52 and the drain wiring 56, and the VDMOSFET reaches an ONstate.

FIG. 8 is a schematic view showing a unit cell having a crystallinestructure of 4H—SiC.

The crystalline structure of 4H—SiC can be approximated by a hexagonalsystem, and four carbon atoms are connected to one silicon atom. Thefour carbon atoms are located on four vertexes of a regular tetrahedronwith a silicon atom located at the center of regular tetrahedronthereof. One of four carbon atoms is located in the direction of the[0001] axis with respect to a silicon atom, while the remaining threecarbon atoms are located on the side of the [000-1] axis with respect toa silicon atom.

The [0001] axis and the [000-1] axis extend in the axial direction of ahexagonal cylinder, and the surface (top face of the hexagonal cylinder)to which the [0001] axis is normal is a (0001) plane (Si plane). On theother hand, the surface (undersurface of the hexagonal cylinder) towhich the [000-1] axis is normal is a (000-1) plane (C plane).

Side surfaces of the hexagonal cylinder to which the [1-100] axis isnormal are (1-100) planes, respectively, and the surface that passesthrough a pair of not-adjoined ridge lines and to which the [11-20] axisis normal is a (11-20) plane. These planes are crystal planesperpendicular to the (0001) plane and the (000-1) plane.

For example, a SiC monocrystalline substrate in which the (000-1) plane(C plane) is a principal plane can be produced by being cut out from aSiC monocrystal ingot in which the (000-1) plane (C plane) is aprincipal plane so that an azimuth error (off-angle) relative to boththe direction of the [1-100] axis and the direction of the [11-20] axisbecomes less than 4° exceeding 0° (preferably, 0.3° or more and lessthan 4°, and more specifically preferably, 1°). The SiC substrate 42 inwhich the (000-1) plane (C plane) is a principal plane (surface 421) canbe obtained by being cut out in this way.

SiC is allowed to grow on the thus obtained SiC substrate 42 accordingto, for example, a chemical vapor growth method, and, as a result, theepitaxial layer 43 is formed.

FIG. 9 is a view for explaining an off-angle of the SiC monocrystallinesubstrate shown in FIG. 7.

The surface 421 of the SiC substrate 42 made of SiC monocrystal is anoff-plane inclined in the direction of the [11-20] axis by θ₃ withrespect to the just (000-1) plane. The angle θ₃ is an off-angle of theSiC substrate 42. More specifically, the angle θ₃ is an angle betweenthe normal [000-1] axis of the just (000-1) plane and the normaldirection n₃ of the surface 421 (off-plane).

As mentioned above, according to the semiconductor device 41, theepitaxial layer 43 is formed on the side of the (000-1) plane (C plane)of the SiC substrate 42, and the off-angle θ₃ of the SiC substrate 42 isless than 4° exceeding 0°. The epitaxial layer 43 is formed by SiC thatgrows from the surface 421 of the SiC substrate 42, and therefore theepitaxial layer 43 has the surface 431 parallel to the surface 421 ofthe SiC substrate 42.

Therefore, the inclination angle of the gate trench side surfaces 462Aand 462B with respect to the (11-20) plane becomes smaller than theangle of the side surface of the gate trench formed in the epitaxiallayer having an off-angle of 4° or more.

More specifically, as shown in FIG. 10A and FIG. 10B, the virtualsurface S₆ is a surface perpendicular to the surface 117 (i.e.,off-plane inclined by off-angle θ₇ with respect to the (0001) plane) inthe conventional semiconductor device 101 (see FIG. 6), and hence isinclined by off-angle θ₇ with respect to the (11-20) plane. Therefore,the inclination angle θ_(4A) with respect to the (11-20) plane of theside surface 118A that is one of the side surfaces 118A and 118B of thegate trench 106 is the sum (for example, 6° or more) of off-angle θ₇ ofthe SiC substrate 102 and taper angle θ₆ of the gate trench 106 (seeFIG. 10B). Inclination angle θ_(4B) with respect to the (11-20) plane ofthe other side surface 118B is a difference (for example, 4° or more)between off-angle θ₇ of the SiC substrate 102 and taper angle θ₆ of thegate trench 106 (see FIG. 10A).

In contrast, as shown in FIG. 11A and FIG. 11B, the virtual surface S₁is a surface perpendicular to the surface 431 (i.e., off-plane inclinedby off-angle θ₃ with respect to the (000-1) plane) in the semiconductordevice 41, and hence is inclined by off-angle θ₃ with respect to the(11-20) plane. Therefore, inclination angle θ_(5A) with respect to the(11-20) plane of the side surface 462A that is one of the side surfaces462A and 462B of the gate trench 46 is the sum (for example, less than5° exceeding 1°) of off-angle θ₃ of the SiC substrate 42 and taper angleθ₁ of the gate trench 106 (see FIG. 11B). Inclination angle θ_(5B) withrespect to the (11-20) plane of the other side surface 462A is adifference (less than 3° exceeding 1°) between off-angle θ₃ of the SiCsubstrate 42 and taper angle θ₁ of the gate trench 106 (see FIG. 11A).

Off-angle θ₃<off-angle θ₇, and therefore the positional relationshipbetween the side surfaces 462A and 462B of the gate trench and the(11-20) plane can be brought close to a more parallel relationship inthe semiconductor device 41 than in the semiconductor device 101including the SiC substrate 102 having an off-angle of more than 4°. Inother words, in the semiconductor device 41, a channel is formed in theregion 451 near an interface between the side surfaces 462A and 462B andthe gate insulating film 47 in the body region 45, and electrons flowalong the side surfaces 462A and 462B. These side surfaces 462A and 462Bcan be brought close to the (11-20) plane that is expected to achievehigh mobility, and therefore channel mobility can be improved.Additionally, an appropriate impurity concentration (carrierconcentration, e.g., 1×10¹⁵ cm⁻³ to 1×10¹⁷ cm⁻³) of the drain region 44of the epitaxial layer 43 can be maintained.

If off-angle θ₃ of the SiC substrate 42 is 0.3° or more and less than4°, the application of a voltage to the gate electrode 48 makes itpossible to fix an electric current (drain current) flowing through achannel formed in the body region 45 and a voltage (gate thresholdvoltage) produced when a drain current begins to flow at substantiallyconstant magnitudes, respectively, in all plane orientations withoutcausing variations by the plane orientation of the side surfaces 462Aand 462B of the gate trench 46. Additionally, the flatness of thesurface 431 of the epitaxial layer 43 can be maintained.

Additionally, the epitaxial layer 43 is formed on the side of the(000-1) plane (C plane) of the SiC substrate 42, and therefore, when thesemiconductor device 41 is produced, the oxidation of the bottom surface461 and the side surfaces 462A and 462B of the gate trench 46 proceedsunder the condition that the oxidation rate of the bottom surface 461 ofthe gate trench and the oxidation rate of the side surfaces 462A and462B satisfy the relational expression: oxidation rate of the bottomsurface 461/oxidation rate of the side surfaces 462A and 462B>1. As aresult, the gate insulating film 47 in which the thickness t₁ of thebottom portion 471 is greater than the thickness t₂ of the side portion472 can be formed. Therefore, the dielectric breakdown of the bottomportion 471 of the gate insulating film 47 can be prevented, andwithstand voltage can be improved by appropriately designing thethickness of the bottom portion 471 of the gate insulating film 47.

Additionally, P-type Poly-SiC whose Fermi level Ef_(Poly) (see FIG. 3A)is closer to the Fermi level of the body region 45 than the Fermi levelof Si is employed as the material of the gate electrode 48. Accordingly,in the MOS (MIS) structure formed of the body region 45, the gateinsulating film 47, and the gate electrode 48, like the relationshipamong the three elements, i.e., the relationship among the body region3, the gate insulating film 6, and the gate electrode 7 shown in FIG.3B, the bend of the energy band of the body region 45 in a state inwhich a voltage is not applied to the gate electrode 48 can be reducedin comparison with a case in which Si is employed as the material of thegate electrode 48. As a result, the threshold voltage of MOSFET of thesemiconductor device 41 can be raised, and an off-leakage current can bereduced in comparison with a case in which Si is employed as thematerial of the gate electrode 48.

The body region 45 has a P-type impurity concentration of 1×10¹⁸ cm⁻³ orless (preferably, 7×10¹⁷ cm⁻³ to 9×10¹⁷ cm⁻³) in a region 451 whosedepth d₁ from the center in the thickness direction of the side portion472 of the gate insulating film 47 is 1000 Å or less. This region 451 islocated near an interface with the gate insulating film 47 and the bodyregion 45, and is a portion where a channel is formed when thesemiconductor device 41 operates. Therefore, channel mobility can bemuch further improved by allowing the region 451 to have a low P-typeimpurity concentration of 1×10¹⁸ cm⁻³ or less.

FIG. 12 is a schematic sectional view of a semiconductor deviceaccording to a fifth embodiment of the present invention. In FIG. 12,the same reference numeral as in FIG. 7 is given to an elementequivalent to each element of FIG. 7. When a structure shown in FIG. 12is described, only differences between the structure of FIG. 7 and thatof FIG. 12 are described, and the element to which the same referencenumeral as in FIG. 7 is given is not described hereinafter.

In the semiconductor device 61 shown in FIG. 12, the gate electrode 62formed on the gate insulating film 47 has a two-layer structureconsisting of a first layer 63 that is formed along the inner surface ofthe gate trench 46 and that is contiguous to the gate insulating film 47and a second layer 64 with which the inside of the first layer 63 isfilled.

The first layer 63 is made of P-type Poly-SiC whose Fermi level iscloser to the Fermi level of the body region than the Fermi level of Si.

The second layer 64 is made of a material different from that of thefirst layer 63. More specifically, the second layer 64 is made of P-typePoly-Si, or N-type Poly-Si, or a metal.

The same operation and effect as the semiconductor device 41 of FIG. 7can be fulfilled by the structure of the semiconductor device 61.

Although the fourth and fifth embodiments of the present invention havebeen described as above, the present invention can be embodied in otherforms.

For example, a structure in which the conductivity type of eachsemiconductor part of the semiconductor devices 41 and 61 is invertedmay be employed. In more detail, in the semiconductor devices 41 and 61,the P-type part may be an N-type, and the N-type part may be a P-type.

Additionally, a structure in which the crystal planes of the frontsurface 421 and the back surface 422 of the SiC substrate 42 areinverted may be employed. In more detail, in the SiC substrate 42, thefront surface 421 may be a Si plane, and the back surface 422 may be a Cplane. In this case, a device is mounted on the Si plane (surface 421)of the SiC substrate 42.

Additionally, for example, materials whose Fermi level is closer to theFermi level of the body region 45 than the Fermi level of Si can bewidely employed as the material of the gate electrode 48 and thematerial of the first layer 63 without being limited to P-type Poly-SiC.

Preferably, materials whose band gap is wider than the band gap of Siare employed as the material of the gate electrode 48 and as thematerial of the first layer 63. Examples of materials suitable as thematerial of the gate electrode 48 and as the material of the first layer63 include SiC doped with N-type impurities or P-type impurities,diamond doped with N-type impurities or P-type impurities, BN (boronnitride) doped with N-type impurities or P-type impurities, AlN(aluminum nitride) doped with N-type impurities or P-type impurities,GaN (gallium nitride) doped with N-type impurities or P-type impurities,GaAs (gallium arsenide) doped with N-type impurities or P-typeimpurities, GaP (gallium phosphide) doped with N-type impurities orP-type impurities, and ZnO (zinc oxide) doped with N-type impurities orP-type impurities.

SiC, diamond, BN, AlN, GaN, GaAs, GaP, and ZnO may be monocrystalline,or polycrystalline, or amorphous. If these materials are polycrystallineor amorphous, there is an advantage in the fact that their films can bemore easily formed than monocrystalline materials.

B, Al, Ga (gallium), and In (indium) can be mentioned as P-typeimpurities with which SiC and diamond are doped.

N (nitrogen), P (phosphorus), As (arsenic), and Sb (antimony) can bementioned as N-type impurities with which SiC and diamond are doped.

Be (beryllium), Mg (magnesium), Ca (calcium), and Sr (strontium) can bementioned as P-type impurities with which BN, AlN, GaN, GaAs, GaP, andZnO are doped.

C (carbon), Si, Ge (germanium), and Sn (tin) can be mentioned as N-typeimpurities with which BN, AlN, GaN, GaAs, GaP, and ZnO are doped.

EXAMPLES

Next, the present invention will be described based on examples andcomparative examples. However, the present invention is not limited bythese examples.

Examples 1 to 6 and Comparative Example 1

(1) Production of TEG (Test Element Group)

An epitaxial layer made of SiC was formed on the side of the C plane ofa 4H—SiC substrate having an off-angle shown in Table 1 below, and thenMOSFET (channel width=160 μm, channel length=0.4 μm) having a structureshown in FIG. 13 was formed in this epitaxial layer, and, as a result,TEGs were produced.

(2) Relationship Between Carrier Concentration and Off-Angle

The carrier concentration of a drift region (drain region) in theepitaxial layer in each TEG mentioned above was measured. The resultsare shown in Table 1 and FIG. 14.

(3) Relationship Between Surface Roughness and Off-Angle of EpitaxialLayer

Root-mean-square roughness (RMS) of the surface of the epitaxial layerin each TEG was measured in conformity to JIS B0601. The results areshown in Table 1 and FIG. 15.

TABLE 1 Carrier concentration RMS Off-angle (cm⁻³) (nm) Example 1 0.31.7 × 10¹⁶ 0.253 Example 2 0.6 1.3 × 10¹⁶ 0.261 Example 3 0.9 1.1 × 10¹⁶0.233 Example 4 1.3 4.7 × 10¹⁵ 0.286 Example 5 1.6 3.9 × 10¹⁵ 0.235Example 6 1.9 3.9 × 10¹⁵ 0.234 Comparative Example 1 0 2.0 × 10¹⁷ 156(4) Evaluation

In Comparative Example 1, the carrier concentration of the drift regionwas excessive (Table 1 and FIG. 14), and the surface of the epitaxiallayer was rough (Table 1 and FIG. 15). In contrast, in Examples 1 to 6,it was confirmed that the carrier concentration of the drift region isappropriate in amount (Table 1 and FIG. 14), and the flatness of thesurface of the epitaxial layer is maintained (Table 1 and FIG. 15).

Example 7 and Comparative Example 2

(1) Processing of Test Substrate

An epitaxial layer made of SiC was formed on the side of the C plane ofthe 4H—SiC substrate having an off-angle of 1° (Example 7) and on theside of the Si plane of the 4H—SiC substrate having an off-angle of 4°(Comparative Example 2), and then a plurality of trench units each ofwhich has a side surface inclined at a predetermined angle with respectto the direction of the [11-20] axis was formed as shown in FIG. 16.Each angle of FIG. 16 shows an inclination angle with respect to the[11-20] axis.

Thereafter, MOSFET having a structure shown in FIG. 13 was formed ateach side surface of each trench unit.

(2) Plane Orientation Dependency of Drain Current (I_(ds))

A drain current I_(ds) generated when a voltage of 20 V was applied tothe gate electrode of each MOSFET mentioned above was measured. Theresults are shown in FIG. 17A and FIG. 17B. In FIG. 17A and FIG. 17B,the drain current I_(ds) of each MOSFET is shown by being standardized(I_(ds)/I_(ds_max)) by the maximum drain current I_(ds).

(3) Plane Orientation Dependency of Gate Threshold Voltage (V_(th))

A gate threshold voltage V_(th) generated when a drain current I_(ds)began to flow to each MOSFET mentioned above was measured. The resultsare shown in FIG. 18A and FIG. 18B. In FIG. 18A and FIG. 18B, the gatethreshold voltage V_(th) of each MOSFET is shown by being standardized(V_(th)/V_(th_max)) by the maximum gate threshold voltage V_(th).

(4) Evaluation

As shown in FIG. 17B and FIG. 18B, in Comparative Example 2, the draincurrent I_(ds) and the gate threshold voltage V_(th) were greatlychanged in magnitude depending on the plane orientation of the trenchside surface, and variations occurred. In contrast, as shown in FIG. 17Aand FIG. 18A, in Example 7, it was confirmed that the drain currentI_(ds) and the gate threshold voltage V_(th) are substantially constant(uniform) in magnitude in all plane orientations of the trench sidesurface.

While the present invention has been described in detail by way of theembodiments thereof, it should be understood that these embodiments aremerely illustrative of the technical principles of the present inventionbut not limitative of the invention. The spirit and scope of the presentinvention are to be limited only by the appended claims.

This application corresponds to Japanese Patent Application No.2009-170154 filed with the Japan Patent Office on Jul. 21, 2009,Japanese Patent Application No. 2009-233777 filed with the Japan PatentOffice on Oct. 7, 2009 and Japanese Patent Application No. 2010-152085filed with the Japan Patent Office on Jul. 2, 2010, the disclosures ofwhich are incorporated herein by reference.

What is claimed is:
 1. A semiconductor device comprising: a firstconductivity type semiconductor layer having an off-angle principalplane that is off angle relative to an internal crystalline plane; atrench formed in the semiconductor layer including a pair of sidesurfaces which face each other, the trench having a tapered shape bynarrowing a distance between the pair of side surfaces from theprincipal plane toward a depth direction of the trench, wherein aninclination relative to the off-angle of one surface of the pair of sidesurfaces is different from that of the other surface of the pair of sidesurfaces; wherein the smaller of the inclinations of the pair ofsurfaces is less than 3° exceeding 1°.
 2. The semiconductor of claim 1,wherein the larger of the inclinations of the pair of surfaces is lessthan 5° exceeding 1°.
 3. The semiconductor device according to claim 1,wherein the semiconductor layer includes an SiC semiconductor layer, thetrench includes a gate trench which penetrates a first conductivity typesource region formed in the SiC semiconductor layer at the principalplane side and a second conductivity type body region formed on anopposite side of the source region from the principal plane, thesemiconductor device further comprises: a gate insulating film formed onan inner surface of the gate trench; and a gate electrode formed insidethe gate trench adjacent the gate insulating film.
 4. The semiconductordevice according to claim 3, wherein one surface in a crystallinestructure of the SiC semiconductor layer is a (000-1) plane, and theoff-angle of the SiC semiconductor layer is less than 4° exceeding 0°.5. The semiconductor device according to claim 4, wherein a bottomsurface of the trench is a surface inclined by the off-angle in adirection of a [11-20] axis with respect to a just (000-1) plane of acrystalline structure of 4H—SiC.
 6. The semiconductor device accordingto claim 4, wherein the off-angle of the SiC semiconductor layer is lessthan 4° exceeding 0.3°.
 7. The semiconductor device according to claim4, wherein the off-angle of the SiC semiconductor layer is 1°.
 8. Thesemiconductor device according to claim 3, wherein a thickness of aportion of the gate insulating film on a bottom surface of the gatetrench is greater than a thickness of a portion of the gate insulatingfilm on a side surface of the gate trench.
 9. The semiconductor deviceaccording to claim 3, wherein the gate electrode is formed by use of asame material as the body region.
 10. The semiconductor device accordingto claim 3, wherein the body region includes a region having an impurityconcentration of 1×10¹⁸ cm⁻³ or less in a portion whose distance fromthe gate insulating film is 1000 Å or less.
 11. A semiconductor devicecomprising: a first conductivity type SiC substrate; a firstconductivity type epitaxial layer formed on the SiC substrate, theepitaxial layer having an off-angle principal plane on an opposite sideof the SiC substrate that is off angle relative to an internalcrystalline plane; a first conductivity type source region formed in theepitaxial layer at the principal plane side; a second conductivity typebody region formed on an opposite side of source region from theprincipal plane; a gate trench formed in the epitaxial layer such thatthe gate trench penetrates the source region and the body region fromthe principal plane, the gate trench including a pair of side surfaceswhich face each other, and the gate trench having a tapered shape bynarrowing a distance between the pair of side surfaces from theprincipal plane toward a depth direction of the gate trench; a gateinsulating film formed on an inner surface of the gate trench; and agate electrode formed inside the gate trench adjacent the gateinsulating film, wherein an inclination relative to the off-angle of onesurface of the pair of side surfaces is different from that of the othersurface of the pair of side surfaces; wherein the smaller of theinclinations of the pair of surfaces is less than 3° exceeding 1°. 12.The semiconductor device according to claim 11, wherein the sourceregion includes an n-type SiC, the body region includes a p-type SiC,and the off-angle of the epitaxial layer is less than 4° exceeding 0°.13. The semiconductor device according to claim 11, wherein the gateinsulating film includes a first portion on a bottom surface of the gatetrench and a second portion on a side surface of the gate trench, andthe first portion is thicker than the second portion.
 14. Thesemiconductor device according to claim 11, wherein the body regionincludes a region having an impurity concentration of 1×10¹⁸ cm⁻³ orless in a portion whose distance from the gate insulating film is 1000 Åor less.
 15. The semiconductor of claim 11, wherein the larger of theinclinations of the pair of surfaces is less than 5° exceeding 1°.